Problemn Set Activity: Writing And Balancing Chemical Equations

Objective:  The purpose of this is to become familiar and competent in writing and balancing chemical equations.  Equations will be balance from first the chemical formula and second from determining the chem formula from the chemical names.

Discussion: Chemical Reactions; the heart of chemistry is the chemical reaction which is responsible for making new substances.  If the reaction is new, then it is similar to inventing a new product.

            Chemical reaction is based on the "Law of conservation of mass".

            Chemical Reaction:

                        Reactant  yield  Product

Reaction of N2 and H2 go NH3

9 H2   +  6 N2   yield   6 NH3  + 3N2  (excess)

Amount reactant = Amount product

The net molecular equation is:

9 H2  (g)  +  3 N2  (g)  yield   6 NH3  (g)

Note:  18 H and 6 N in each side!!

In general a chemical reaction can be expressed as:

a A  +  b B   yield    c C  +    d D

where

a, b - coefficient of reactant (how many).

c, d - coefficient of product (how many).

# atoms (reactant) = # atoms product :  Balance

Example 1:

Mg and O2 to MgO

 

To balance: 

        Mg  +   O2  -->  MgO2    No!! (never since MgO2  is different from MgO)
       Mg  +  O2  -->  2MgO     Oxygen okay,   Mg not okay

      Now oxygen okay and Mg okay.

                                          

        


 

 

Example #2:

            Summary :  Balancing chemical reaction

            • Once chemical formula of specie is determine, do not change subscript.

            • Assign coefficient of 1, to most complicated molecule

            • Balance Homogeneous atomic molecules last i.e., H2, N2, O2.

            • Balance cation/anion as a single unit (i.e., if occurs in reactant and product unchanged)

            • Convert fractions to integers.

            Try the following as an exercise.

 

Example # 3:  Balancing a double displacement (metathesis)

Consider the reaction: Mg(NO3)2 + KOH Products
What are the products formed and how can the equation be balanced and?

Double Displacement Equation Scheme

Solubility Rules

3 ways of writing the chemical reaction for a double displacement reaction:

a) Molecular equation; Equation showing overall reaction stoichiometry, but not the form of reactant or product in solution.  (The forms are implies by the phase-subscript).

b) Complete ionic equation; Equation showing ions that are strong electrolyte.

c) Net ionic equation; Equation showing only substances undergoing chemical changes.

Summary:

a) Molecular equation:
         Mg(NO3)2(aq) + 2KOH(aq) 2KNO3(aq) + Mg(OH)2 (s)

b) Complete ionic equation:
          Mg2+(aq) + 2 NO3- (aq) + 2 K+(aq) + 2 OH-(aq) 2 K+ (aq) + 2 NO3- (aq) + Mg(OH)2 (s)

c) Net ionic equation:
         Mg2+(aq) + 2 OH-(aq)  Mg(OH)2 (s)

Example #4 . Balancing a neutralization reaction ( a form of double displacement)

Acids - provides H+

Base  - provides OH-

Water is produce and a salt (ionic compound) 

salt :  (cation - anion)

acid in water;  at a molecular level. - (Aqueous chemistry)

 

Try the folloinw as exercise.

#5    Sodium oxide and water forms sodium hydroxide

#6    Ferric(III)  Sulfide & molecular oxygen yields ferric(III) oxide & sulfur dioxide.

#7  C4 H10 (g)  +   O2 (g)      H2O(g)  +  CO2 (g)  (combustion)

       In a combustion reaction, water,  CO2 and energy are always produced.

#8  H2SO4 (aq)  +  Ca(OH)2 (aq)   H2O(l)  +  CaSO4 (s)

 
 

1. Combination Reaction

         A  +  B g  AB

   Example:

   i)   __ SO3  +  __ H2O   g   __ H2SO4

   ii)  __ P  +  __ S g __ P2S5

   iii)  __ N2  +  __ H2 g __ NH3

 

2. Decomposition Reaction

         AB   g  A  +  B

Example:

   i)  __ H2CO3 g __ H2O   +   __ CO2

   ii)  __ KClO3 g __ KCl  +  __ O2

 

3. Single Displacement Reaction:

         A  +  BC g AC  +   B

   Example:

         i)  __ Zn  +  __ Cu(NO3)2   g __ Zn(NO3)2  +  __ Cu

         ii)  __ Ca  +  __ H2O g __ Ca(OH)2  +  __ H2

 

4a.  Double Displacement Reaction:

         AB  +  CD g AD  +   CB

     Example:

     i)   __ Ba(NO3)2  +  __ H2S g __ BaS  +  __ HNO3

     ii)  __ MgCl2  +  __ Na3PO4 g __ Mg3(PO4)2  +  __ NaCl

 

4b. Acid Base (Double Displacement) Reaction:

         HA  +  MOH g H2O  +   MA

     Example:

     i)   __ HNO3 +  __ Ba(OH)2 g __ H2O +  __ Ba(NO3)2

     ii)  __ HCl  +  __ NH4OH g   __ H2O   +  __ NH4Cl

 

5.  Combustion Reaction

         CxHy  +  O2 g CO2  +  H2O

     Example:

     i)  __ C4H10 +  __ O2 g __ CO2  +  __ H2O

     ii)  __ C6H12O6  +  __ O2 g   __ CO2  +  __ H2O


WWW Links to balancing chemical reactions (Accessed Jan 04)
1. http://198.234.204.51/joe_didomenico/chem_honors/chpt6-7-8/chpts6_7_8_rxns.htm
2. http://home.downloadatoz.com/chemical-equation-expert/
3. http://mslc.rutgers.edu/nsfgk12/Lessons%20and%20Activities/Lesson%20Plans/Physical%20Sciences/Double%20Displacement.htm
4. http://chemistry.about.com/cs/generalchemistry/a/aa072103a.htm
5. http://antoine.frostburg.edu/chem/senese/101/reactions/faq/predicting-products-nicl2-nh42s.shtml
 

 
 
Problem Set Activity:         Writing And Balancing Chemical Equations
___/ ___ Score
Name (Last)_______________(First)_______________ Date _______

A.  Balance the following equations:

 

Example: 2 H2 + 1 O2 g 2H2O Sum of coefficient = 2 + 1 + 2 = 5
1.         

Sum of coefficient = 2 + 1 + 2 = __

 
2.         

Sum of coefficient = 2 + 1 + 2 = __

 
3.        

Sum of coefficient = 2 + 1 + 2 = __

 

4.         

Sum of coefficient = 2 + 1 + 2 = __

 

 

B.  Complete and balance the following double displacement rxn equations (assume all reactions goes to completion):

 

1.            

 

 

2.            

 

 

3.            

 

 

4.            

 

 

 

 

C.  Complete and balance (Combination, Decomposition, Single displacement, Double displacement)

1.                _ K(s)  +  _ Cl2 (g)     

                                            (potassium chloride)

 

2.                _ Zn (s)   +  _ O2 (g) 

                                                  (zinc oxide)

 

3.                _ BaO (s)   +  _ H2 O(l)   

                                               (barium hydroxide)

 

4.                _ SO3(g)    + _H2 O (l)   

                                                  (sulfuric acid)

 

5.                _ Ni (s)    +  _ HCl (aq)     

                                         nickel(II)chloride and hydrogen

 

6.                _ Pb (s)    +  _ AgNO3(aq) 

                                             lead(II)nitrate and silver

 

7.                _ (NH4)2 SO4(aq)   +  _ KOH (aq)  right arrow

                       ammonium hydroxide and potassium sulfate

 

8.                _ ZnCl2 (aq)   +  _ (NH4)2 S (aq) 

                           (zinc sulfide and ammonium chloride)

 

 

D. Beneath each word equation write the formula equation and balance it.  Keep in mind those elements that exist as diatomic molecules:

1.    Sulfur (s)  +  Oxygen (g)    Sulfur dioxide (g)

 

2.    Zinc (s)   +  Sulfuric acid (aq)      Zinc sulfate (aq)   +  Hydrogen (g)

 

 

3.    Carbon (s)   +  Oxygen (g)    Carbon dioxide (s)

 

 

4.    Hydrogen (g)    +  Oxygen (g)      Water (g)

 

 

5.    Aluminum (s)   +  Oxygen (g)     Aluminum oxide (s)